1. Field of the Invention
This invention relates to a densitometer, and more particularly to a densitometer to be used for measuring the transmission optical density and the reflection optical density of various kinds of sheet material such as photographic film, photographic paper, printed paper or the like.
2. Description of the Prior Art
The Macbeth model densitometer (made by Kollmorgen Corp.) is one of this type of densitometer adapted to measure the optical density of a photographic material. The Macbeth densitometer includes a transmission densitometer and a reflection densitometer which are separately used. The reflection densitometer employs a double cylinder structure in which a light source is provided within the inner cylinder to illuminate a sample at a right angle with respect to the surface thereof and the light reflected by the sample in the direction inclined at 45.degree. with respect to the surface thereof is collected by a mirror attached to the inner surface of the outer cylinder and directed to a photodetector. When measuring the optical transmission density, a measuring head is moved as a whole in the direction of the optical axis and put into contact with the surface of the sample. The transmission densitometer of this type has a condenser lens at the top of a lens barrel thereof, which is movable in the direction of the optical axis. When measuring, the top of the lens barrel is put into contact with the surface of a sample and a parallel flux of light of illumination is irradiated on and through the sample from the back thereof.
The above described Macbeth densitometer is disadvantageous in that the transmission optical density and the reflection optical density must be measured with separate densitometers. In view of this defect, a Quantascan densitometer (made by Quantametric Devices Inc.) has been developed which is able to measure both the transmission and reflection optical density with a single densitometer. The Quantascan densitometer employs a double cylinder structure in which a conical mirror provided on the inner surface of the outer cylinder reflects the illumination light from a light source toward the surface of a sample and the light reflected by the surface of the sample is guided through the inner cylinder to a photodetector, and the sample is illuminated from the back thereof with a condensed flux of light and the light transmitted through the sample is guided through the inner cylinder to a photodetector. The Quantascan densitometer has a defect in that the measuring head including the double structure cylinder is not movable, and accordingly, the density is not accurately measured when the surface of the sample is light diffusive or when the thickness thereof changes. When the reflection density is measured, the area of the sample illuminated and measured varies as the thickness of the sample varies. When the transmission density is measured, the distance between the surface of the sample and the lens which receives light passing through the sample varies as the thickness of the sample changes. In other words, as the thickness of the sample increases, the angle of light incident to the light receiving lens increases and accordingly the amount of light received is increased, thus the density measured is lowered. Further, since the lower end of the lens barrel is separated from the surface of the sample, the measurement will be inaccurate if the surface of the sample is curled or waved.